1. Field of the Invention
This invention relates to rush current prevention technology and, more particularly, to a technology for prohibiting the production of a penetration current in circuitry when an input signal is at an indefinite level.
2. Description of the Related Art
A system is widely known wherein back-up power is supplied to some circuit such as a memory when the power source is off. In such a system, backed-up circuits and non backed-up circuits coexist in the circuitry and an output signal from the non backed-up circuits goes to an indefinite level when the power source is turned on and off. Where the system is of a type in which an output from the non backed-up circuit is supplied to the backed-up, penetration, surge current or rush current flows through the backed-up circuit when the power source is turned on and off. This increases power consumption of the back-up battery and, hence, shortens a time period available for back-up. Furthermore, rapid changes in current flow decrease the back-up voltage, so that it is likely that a normal back-up voltage is not obtained.
It is known that, when the output signal changes to an indefinite level, a large rush current flows through a CMOS transistor. Therefore, the above drawback become particularly critical where the backed-up circuit is formed of a CMOS transistor.